Vehicles today use, for example, urea as reductant in SCR (selective catalytic reduction) systems which comprise an SCR catalyst, in which catalyst said reductant and NOx gas can react and be converted to nitrogen gas and water. Various types of reductants may be used in SCR systems. AdBlue is an example of a commonly used reductant.
One type of SCR system comprises a container for a reductant. The SCR system may also have a pump adapted to drawing said reductant from the container via a suction hose and to supplying it via a pressure hose to a dosing unit situated adjacent to an exhaust system of the vehicle, e.g. adjacent to an exhaust pipe of the exhaust system. The dosing unit is adapted to injecting a necessary amount of reductant into the exhaust pipe upstream of the SCR catalyst according to operating routines stored in a control unit of the vehicle. To make it easier to regulate the pressure when no or only small amounts are being dosed, the system also comprises a return hose which runs back from a pressure side of the system to the container. This configuration makes it possible to cool the dosing unit by means of the reductant which, during cooling, flows from the container via the pump and the dosing unit and back to the container. The dosing unit is thus provided with active cooling. The return flow from the dosing valve to the container may be substantially constant.
In today's vehicles, cooling of the dosing unit is assured by having a rather high least permissible volume of reducing agent in the container. Cooling of the dosing unit may thus be assured even during extreme outdoor temperatures. A disadvantage of this technique is that a relatively large proportion of the reducing agent held in the container cannot be used for dosing into the exhaust pipe for use in the SCR catalyst. This means that in practice said reducing agent acts to some extent as ballast, resulting in a number of adverse effects, e.g. that the vehicle's total load capacity is less than it might be, both in weight and in volume.
WO 00/21881 describes a temperature-controlled system for injecting urea solution into an exhaust system whereby the urea solution is pumped from a tank in sufficient mass flow not only to make sufficient injection possible but also to cool an associated injector to a sufficiently low temperature. The coolant urea solution may be led back to the tank. The system comprises volume meters and temperature sensors in the tank. When deviations occur, the system can halt the circulation of the urea solution. This entails the disadvantage that an undesirable temperature may occur at the injector.
There is thus a need to improve current SCR systems in order to reduce or eliminate the above disadvantages.